automated measurement of contract CPU/computron/memory usage

[warner]

What is the Problem Being Solved?

As discussed in #4219 (comment) , before we can confidently enable per-delivery computron limits, we need to know that none of our contracts are at risk of exceeding them.

That means some sort of automated test which pays close attention to the metering results of startVat/buildRootObject (contract startup), plus a few typical deliveries. It should also track how large the contract bundles are (in bytes), to compare against transaction size limits.

Description of the Design

I'm guessing this would fit into @mhofman 's loadgen framework, or the automation that @arirubinstein has built.

[warner]

The reporting on this should give us an alert if the usage comes within 50% of whatever limits we impose on vats in the chain.